U.S. Pat. No. 4,478,749, issued Oct. 23, 1984, describes antibiotics comprising a .beta.-lactam nucleus having in the 3-position an acylamino substituent and in the 1-position an activating group of the formula ##STR2## wherein R'.sub.a is hydrogen, alkyl, substituted alkyl, phenyl or substituted phenyl, and R'.sub.b is hydroxy, alkoxy, (substituted alkyl)oxy, phenyloxy, (substituted phenyl)oxy, alkyl, substituted alkyl, phenyl, substituted phenyl, heteroaryl, amino, substituted amino, alkylthio, (substituted alkyl)thio, phenylthio, (substituted phenyl)thio, 1-(ethoxycarbonyloxy)ethoxy, 1,3-dihydro-3-oxo-1-isobenofuranyloxy, ##STR3## wherein R'.sub.c is hydrogen or alkyl, R'.sub.d is alkyl or phenyl, R'.sub.e is hydrogen, methyl or phenyl and R'.sub.f is hydrogen or together with R'.sub.e is --(CH.sub.2).sub.3 -- or --(CH.sub.2).sub.5 --.